A DPF is provided in an exhaust pipe in order to purify exhaust gas from a diesel engine by removing particulate matter (to be referred to hereafter as PM) from the exhaust gas. The DPF collects the PM in a honeycomb structure formed from a porous ceramic. When the collected PM accumulates excessively, circulation of the exhaust gas is impaired, but by increasing an exhaust gas temperature, the PM collected in the DPF can be incinerated and removed. This operation is known as DPF regeneration.
In a conventional method for increasing the exhaust gas temperature to a temperature at which the PM can be incinerated, an oxidation catalyst (to be referred to hereafter as a DOC) constituted by platinum or the like is disposed upstream of the DPF, and the exhaust gas temperature is raised to an activation temperature of the DOC by increasing a fuel injection amount of a multi-injection, in which fuel injection is performed a plurality of times to obtain propulsive force in the engine. Next, a DPF regeneration fuel injection (a post injection) is performed at an appropriate crank angle such that hydrocarbon (to be referred to hereafter as HC) is supplied to the DOC by the post injection, and the exhaust gas temperature is raised by oxidation heat from the HC. When the post injection is performed, however, oil dilution, in which the injected fuel intermixes with lubricating oil of the engine and thereby diluted, occurs. Further, when unburned fuel from the post injection intermixes with exhaust gas recirculation (to be referred to as EGR hereafter), a performance of an EGR cooler deteriorates and a defect occurs in a piston ring.
Hence, in recent years, a method of supplying HC to the DOC by providing a new device in the exhaust pipe and performing an exhaust pipe injection to inject fuel into the exhaust pipe such that the exhaust gas temperature is raised by the oxidation heat of the HC has been investigated.